Make grepview's GrepFindFilesThread thread-safe

Testing done:

1. All 22 test_findreplace's test cases still pass.
2. Applied GrepFindFilesThread-benchmark.patch and run Find/Replace on several directories and projects at KDevelop master and at this branch. There is no noticeable performance difference (< 1%). Furthermore, I have benchmarked GrepFindFilesThread with all checking of shouldAbort() removed and at !157 (merged) => the performance in these two cases was within 1% as well. This means that the cost of loading the atomic variable in the loops is negligible compared with the other work done in these loops.

See commit messages for details.